The following terms used in this patent document have generally accepted meanings in engineering literature and will not be specifically defined herein: resistance, component, circuit, electrons and electronic, capacitor and capacitive, control, signal, voltage, current, power, energy, frequency, radio-frequency (RF), sensor, actuator and actuation, electrostatic, electromagnetic, piezoelectric, electromechanical, thermomechanical, all terms of the S.I. and English unit systems, and all materials used in science and industry.
Other terms have meanings in the context of this document that will be made clear. The description of the drawings and detailed description portions of this document include some of these precise terms that describe numbered elements of the drawings as they occur in the text. For the purposes of this patent document, the following terms are now defined:
An actuation is defined as the action of opening or closing a relay or other switching device. An actuator is defined as the energy conversion mechanism responsible for actuation.
An armature is defined as an element that is deflected or moved by an actuator in order to move from one state to another state in a device with multiple stable states.
A μm, micron, or micrometer is a unit of length equal to one-one-thousandth of a millimeter.
Microfabrication is a fabrication method of defining components delineated through photolithographic techniques made popular by the integrated circuit developer community.
Micromachining is the action of delineating a microfabricated element that has been photolithographically defined, often performed by an etching process using acids or bases or by physical means such as ion milling or photoresist lift-off techniques.
MEMS and MEMS devices are Microfabricated ElectroMechanical Systems, which denotes a manufacturing technology that uses microfabrication techniques to develop miniaturized mechanical, electromechanical, and thermomechanical components. MEMS devices in this context include contact devices, which include a subset of physical and electrical contact devices such as switches, relays, and switchable capacitors.
Many MEMS devices based on cantilevers been incorporated into a wide variety of devices used in everyday life such as airbag accelerometers, disk drives, and chemical sensors. A staggering variety of various cantilever manufacturing processes, technologies, and architectures have been employed in the literature and in products throughout the past 20 years, with most of these devices having low forces available for actuation and restoration.
Certain switching devices have multiple stable states, and are typically driven by control signals to cause them to assume a particular one of the possible stable states. In most devices of this type, one state is a passive state, which is its “natural” condition when no control signals are applied. When an active state is desired, a drive control signal is applied to an actuator(s), which deflects an armature(s) until mechanical limitations prevent further deflection. Once configured into a second state, it may be desirable to hold the switching device in its second state for a period of time using either passive or active latching technologies. In many contact devices, removal of the latching control signal or condition can then send the switching device back to its passive state.
Conventional MEMS contact devices can encounter a problem during the restoration part of the operating cycle described previously, as there can be forces in place during contact that are sufficient to overpower the restoring forces available from the deflected armature. Many commonly configured cantilever devices stick to its lower substrate when it was supposed to be free. This problem arises due to a variety of surface effects such as metallic cold welding, solid bridging, Van der Waal's forces, capillary forces, etc., and was dubbed the non-technical moniker “stiction” early in the MEMS industry.
Engineering around the adhesion problem by selecting careful geometries and materials is difficult for typical cantilever switch architectures, as the spring strength of the cantilever itself becomes the limiting factor. A strong spring, such as that with a spring constant (k) larger than 10 N/m (relative strength being depending on the device geometry and operation), can be designed to break a switch contact, or pull the cantilever away from surface bonding effects. A strong spring is also harder to move in the first place, however, which means larger forces are required to move it. Increasing the available forces and motive power available from the actuation mechanism is the first engineering solution to the problem of using a stronger spring, but limitations of available voltage, current, or size quickly challenges the designer.
Conventional techniques for addressing adhesion issues focus on altering the physical chemistry or surface effects of micro-scale mating surfaces, or by reducing the physical surface area available for these nano-scale forces to operate upon.
Although the problem of undesirable surface adhesion from manufacturing and during operation has been addressed in a number of ways, the problem of MEMS cantilevers sticking to other surfaces is still commonly seen as a regular yield factor or operational failure mechanism in industry. The additional complexity of many of the solutions discussed adds additional cost. The operational adhesion problems are particularly relevant for MEMS devices that are supposed to stick to another surface in one state, and are supposed to release from that surface in another state. This is most commonly seen in switching devices such as MEMS switches and relays, which make and break Ohmic or capacitive contact between load signal electrodes during operation.
Some MEMS arrangements incorporate active solutions to the problem of devices that are stuck to the other surface. One typical example is to provide for a drive electrode on the opposing face of a typical electrostatically-actuated device, so that a cantilever can be actuated closed and also actuated open from the opposite side. This provides increased speed and force as compared to the passive restoring forces of deflected springs more typically found in MEMS devices.
An equivalent solution is very commonly used across the range of actuation types, including electrostatic, electromagnetic, thermal, and piezoelectric devices. The design penalty for multi-actuator device architectures can be significant, however, in terms of increased complexity of manufacture and operation, increased cost due to complexity and yield, and engineering limitations due to processing compatibility constraints. For electromagnetic and thermal actuation mechanisms, another penalty is increased power consumption, as these require power to operate both closed and open, instead of using power only in one direction and using passive restoring springs for the other.
None of the conventional solutions is optimal for MEMS devices that are simple to manufacture, make surface contact as part of their normal operation or environmental tolerance requirements, and require a high restoring force through passive operation.